Which substance is used as an oxidising agent in a nickel-cadmium cell?

Standard electrode potentials for a few half cells are mentioned below:
$E^0_{Cu^{2+}/Cu} = 0.34 \ V, E^0_{Zn^{2+}/Zn} = -0.76 \ V$
$E^0_{Ag^{+}/Ag} = 0.80 \ V, E^0_{Mg^{2+}/Mg} = -2.37 \ V$
Which one of the following cells gives the most negative value of $\Delta G^0$?

Which cell differs from the fundamental principle point of view?

For the reduction of silver ions with copper metal,the standard cell potential was found to be $+ 0.46 \ V$ at $25^oC.$ The value of standard Gibb's energy,$\Delta G^o$ will be .......... $kJ.$
$(F = 96500 \ C \ mol^{-1})$

The Gibbs energy change (in $J$) for the given reaction at $[Cu^{2+}] = [Sn^{2+}] = 1 \, M$ and $298 \, K$ is:
$Cu_{(s)} + Sn^{2+}_{(aq)} \rightarrow Cu^{2+}_{(aq)} + Sn_{(s)}$
$(E^{\circ}_{Sn^{2+}|Sn} = -0.16 \, V, E^{\circ}_{Cu^{2+}|Cu} = 0.34 \, V, F = 96500 \, C \, mol^{-1})$

At $298 \ K$,the standard electrode potentials of $Cu^{2+}/Cu$,$Zn^{2+}/Zn$,$Fe^{2+}/Fe$ and $Ag^{+}/Ag$ are $0.34 \ V$,$-0.76 \ V$,$-0.44 \ V$ and $0.80 \ V$,respectively. On the basis of standard electrode potential,predict which of the following reactions cannot occur?